The present invention relates to communication networks. More particularly, the present invention relates to a method and apparatus for transporting private line traffic over an ATM network.
Within a telecommunication network, xe2x80x9cprivate linexe2x80x9d circuits (TDM lines) may be used to transport information, including voice and/or data traffic. A private line connection between two points may be used, for example, by a business to connect geographically distant offices. The private line connection is reserved, and therefore the network provider can assure a high Quality of Service (QOS) in terms of bandwidth and delay. Although private line traffic may not need such a high QOS, existing customer agreements and equipment often require them. A T-1 circuit is one example of a private line circuit and provides a maximum transmission speed of 1.544 megabits per second (Mb/s).
In order to provide this QOS, private line private line traffic is traditionally transported using a Synchronous Transfer Mode (STM) network. A network using Time Division Multiplexing (TDM) is one example of an STM network. Using TDM, each channel of private line traffic is assigned a specific time period, or TDM channel, configured to let the channel carry a desired maximum rate of data transmission. In this way, the STM network provides a high QOS because each TDM channel, by design, can handle the maximum amount of data information. As a result, data information is generally not lost or delayed. If, however, less than the maximum amount of data information is being sent over a TDM channel, a number of the channel""s assigned time periods are not used, and bandwidth is therefore wasted when no data is being transmitted.
It is also known that private line traffic can be transported via an Asynchronous Transfer Mode (ATM) network. An ATM network uses dedicated-connection switching technology that organizes digital data into 53-byte cells and transmits them over a medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and may be queued before being multiplexed with other cells, from other channels, over a single line, or xe2x80x9clink.xe2x80x9d Because ATM networks are more easily implemented by hardware (rather than software), faster processing speeds are possible. In addition, ATM networks allow for more efficient bandwidth use because different services, such as voice and data, can be statistically multiplexed over the same link.
Generally, TDM circuits are transported over an ATM virtual circuit using an ATM Adaptation Layer (AAL). An AAL adaptation layer merely packages higher layer information, such as the T1 or E1 circuit information, into the contents of the 53-byte ATM cell. A number of these virtual circuits are then combined for transport over an ATM network link, such as over a single ATM network xe2x80x9cpipe.xe2x80x9d
To maintain the high quality traditionally associated with STM networks, the AAL1 adaptation layer is used together with Constant Bit Rate (CBR) service (together known as xe2x80x9ccircuit emulationxe2x80x9d). As with TDM, the CBR circuit emulation approach provides a constant guaranteed rate of transfer. That is, a CBR connection allocates a channel enough bandwidth to support the corresponding STM rate. In this way, CBR circuit emulation provides a QOS similar to that of an STM network, but does not provide any statistical multiplexing benefits since cells are still used even when no information is being transported. In other words, with circuit emulation the excess bandwidth that is not used by a customer is not available in the ATM network for other services. This may be a significant amount of unused bandwidth, especially during non-business hours.
In view of the foregoing, it can be appreciated that a substantial need exists for a method and apparatus to transport private line traffic over an ATM network that allows for statistical multiplexing benefits while still maintaining a high QOS.
The disadvantages of the art are alleviated to a great extent by a method and apparatus for transporting private line traffic over an ATM network. A first plurality of TDM private line traffic links, such as T1 or E1 circuits, are multiplexed to create a first rt-VBR virtual circuit such that the bandwidth of the first rt-VBR virtual circuit is not limited. A second plurality of TDM private line circuits are multiplexed to create a second rt-VBR virtual circuit such that the bandwidth of the second rt-VBR virtual circuit is not limited. This may be done by AAL2 multiplexing with substantially large SCR, PCR and MBS values and the removal of T1 frames that do not contain data (that is, frames that contain xe2x80x9cfillerxe2x80x9d bytes). The first and second rt-VBR virtual circuits are combined for transport over a link in the ATM network. An overload control process may be performed based on the ATM network link utilization.
With these and other advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.